1. Field of the Invention
The present invention relates to an electrochemical test cell and a method to screen combinations of materials for compatibility and performance in an electrochemical cell.
2. Discussion of the Background
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/eq and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Production of a battery having an anode based on magnesium, requires a cathode which can reversibly adsorb and desorb magnesium ions and an electrolyte system which will efficiently transport magnesium ions.
The electrochemical behavior of a magnesium electrode in a polar aprotic electrolyte solution was reported by Lu et al. in the Journal of Electroanalytical Chemistry (466 (1999) pp 203-217). These authors concluded that the electrochemical behavior of Mg is different from that of Li in polar aprotic electrolyte solutions. Their investigation showed that in contrast to the case of lithium electrodes, surface films which form on the Mg electrode in the aprotic solvents do not transport Mg ions. Therefore, conventional electrolyte systems are not suitable for a cell having a magnesium anode. Since Mg ion transport is an essential requirement for any electrochemical cell based on a magnesium anode, other electrolyte systems have been investigated.
Gregory et al. (J. Electrochem. Soc., 137 (3), March, 1990, 775-780) reported electrolyte systems of alkylmagnesium halide-organoboron complexes in an ether solvent. Also reported were alkylmagnesium halide solutions to which aluminum halides were added. Mg dissolution and plating at very high current efficiencies, giving bright crystalline Mg deposits were obtained in these systems. However, a suitable cathode material, compatible with the electrolyte system was not reported. The most commonly used magnesium electrolyte to date is an organometallic material such as phenyl magnesium chloride/aluminum chloride in tetrahydrofuran. However, these electrolyte mixtures are not likely to be of practical commercial utility due to air and moisture sensitivity characteristic of such Grignard materials. Moreover, the phenyl magnesium chloride/aluminum chloride electrolyte has limited anodic stability, and significantly, such materials are highly nucleophilic and intrinsically strong reducing agents. This chemical reactivity character is problematic, because to construct an electrochemical cell employing a Grignard type electrolyte, a cathode material which is essentially chemically inert to the Grignard is required.
Sulfur is of low cost and low molecular weight and could be used as a cathodic material in combination with a magnesium anode to provide a high capacity, safe and economic battery, potentially suitable for use in EV. However, the organometallic electrolytes employed in the above magnesium electrolyte systems are highly reactive with sulfur and are known to directly react with sulfur to form sulfides by nucleophilic attack (The Chemistry of the Thiol Group, Pt 1; Wiley, New York, 1974, pp 211-215).
Therefore, in order to produce a Mg/S battery, a new electrolyte system which meets all the requirements for magnesium ion transport described previously while having low or no chemical reactivity toward sulfur is required.
Investigation of such systems is typically complex and problematic in that the ultimate evaluation of suitability and performance is conventionally dependent upon construction of a coin cell which is time consuming and subject to error in construction which may lead to misleading results. Moreover, conventional systems often require utilization of substitute electrodes of a material different from the test material when the test combination is not chemically compatible. This is especially the case in the investigation of potential electrochemical systems wherein the electrolyte is chemically reactive with metal parts, such as a stainless steel current collector used in conventional cells. Therefore, there is a need for a system and method to evaluate various combinations of anode, cathode and electrolyte materials which does not involve the complex fabrication of a coin cell or a system having substitute working electrodes, requires less time than conventionally known methods and provides adequate demonstration of the feasibility and potential electrochemical performance of the test combination as a power source.
The testing methods conventionally known do not meet the described need for a simple, straight forward and accurate test system and method.
For example, U.S. Pre-Grant Publication No. 2010/0310933 to Jiang describes an electrolyte for a cell having a magnesium or magnesium alloy anode and a cathode having an active material which includes iron disulfide. The electrolyte comprises a magnesium salt dissolved in an organic solvent and an additive to retard buildup of a passivation coating on the magnesium anode surface. In preparation of a test cell system, Jiang presses a mixture of iron disulfide and teflonized acetylene black onto an aluminum collector sheet which is therefore in contact with the electrolyte.
U.S. Pre-Grant Publication No. 2009/0226809 to Vu et al. describes a cathode for a lithium-sulfur battery (Abstract). A metal oxide selected from Group I and II metals is included in the composition of a sulfur cathode composition [0012]. The anode contains lithium and the electrolyte described is composed of a lithium salt in a nonaqueous solvent system [0032].
U.S. Pre-Grant Publication No. 2008/0182176 to Aurbach et al. describes an electrochemical cell having a magnesium anode and an intercalation cathode having a modified Chevrel phase. The Chevrel phase compound is represented by the formula Mo6S8-ySey (y is greater than 0 and less than 2) or MxMo6S8 (x is greater than 0 and less than 2). This material is coated onto a metal such as aluminum as a current collector. The electrolyte is represented by the formula Mg(AlRxCl4-x)2 and/or (MgR2)x-(AlCl3-nRn)y wherein R is methyl, ethyl, butyl, phenyl and derivatives thereof, n is greater than 0 and lower than 3, x is greater than 0 and lower than 3 and y is greater than 1 and lower than (Claim 3) in an ether solvent. Therefore, both magnesium and the metal of the cathode collector are in contact with the electrolyte.
U.S. Pat. No. 7,316,868 to Gorkovenko describes an electrochemical cell having a lithium anode, a cathode of an electroactive sulfur containing composition and a nonaqueous electrolyte containing a lithium salt and a solvent mixture of dioxolane and one or more of 1,2-dialkoxyalkanes of 5 or 6 carbons and 1,3-dialkoxyalkanes of 5 or 6 carbon atoms (Claim 1). Electroactive sulfur compounds include elemental sulfur and organic compounds having sulfur and carbon atoms (Col. 4, lines 10-26). Cathodes were prepared by coating a current collector such a s aluminum with the sulfur containing material. Therefore, both lithium and the metal of the cathode collector are in contact with the electrolyte.
U.S. Pat. No. 7,189,477 to Mikhaylik describes an electrochemical cell having a lithium anode, a cathode of a sulfur containing material and an electrolyte system composed of a lithium salt (Col. 4, lines 5-22) and a non-aqueous oxygen containing organic solvent selected from acyclic ethers, cyclic ethers, polyethers and sulfones. Cathodes were prepared by coating a current collector such a s aluminum with the sulfur containing material. Therefore, both lithium and the metal of the cathode collector are in contact with the electrolyte.
U.S. Pat. No. 7,029,796 to Choi et al. describes lithium sulfur battery having a cathode of an agglomerated complex of sulfur and a conductive agent particles (Claim 1). A solid or liquid electrolyte can be employed and a liquid electrolyte is a nonaqueous organic solvent and a lithium salt (Col. 8, lines 43-58). The agglomerated complex of sulfur is coated on a metal current collector such as stainless steel, aluminum or copper among others. Therefore more than one metal contacts the electrolyte.
U.S. Pat. No. 6,733,924 to Skotheim et al. describes lithium sulfur battery wherein the lithium is protected by a surface coating of a metal such as copper, magnesium, aluminum, silver, etc. (Col. 12, lines 25-28). The electrolyte may be comprised of ionic salts in a non-aqueous solvent, gel polymer or polymer. Ionic electrolyte salts are lithium salts (Col. 15, line 26 to Col. 16, line 27).
U.S. Pat. No. 6,420,067 to Yoshioka describes a hydrogen storage negative electrode being a Mg alloy of Ni, Zn, and Zr (Abstract). The positive electrode is composed of a metal oxide (Col. 3, lines 17-19) and an aqueous electrolyte Col. 7, lines 16-18). Conventional cathodes prepared by coating a metal current collector are employed. Therefore, multiple metals contact the electrolyte.
JP 2004 265675 to Hideyuki et al. describes a nonaqueous electrolyte battery having a magnesium containing anode, a cathode containing sulfur and a reference electrode containing lithium. The reference electrode is apparently necessary because the Mg(TFSI)2-γ-butyrolactone electrolyte employed is not compatible with Mg electrode. Hideyuki does not describe a magnesium sulfur cell where the only metal in contact with the electrolyte is the anode metal.
U.S. Pat. No. 7,361,257 to Wang et al. describes a complex device containing a rotating disk electrode and method to evaluate an electrochemical reaction. The device contains an electrochemical cell containing working electrodes of metals and metal alloys which are stable to high temperatures and does not describe a test system cell where only one metal is in contact with the electrolyte.
U.S. Pat. No. 5,686,201 to Chu describes a positive electrode containing sulfur, an electronic conductor and an ionic conductor. The positive electrode is prepared by depositing a sulfur containing slurry onto a metal current collector. Therefore a test cell wherein only one metal is in contact with the electrolyte is not disclosed or suggested.
U.S. Pat. No. 5,425,870 to Stein describes an instrument for measuring the throwing power, electrochemical efficiency and operating current density of an electrolytic bath for electrolytic processing of materials. Multiple electrodes of different metal construction are contained in the meter.
As indicated, none of these references describing conventional systems discloses or suggests a system and method to evaluate various combinations of anode, cathode and electrolyte materials which does not involve the complex fabrication of a coin cell or a system having substitute working electrodes, and provides adequate demonstration of the feasibility and potential electrochemical performance of the test combination as a power source.